Butterfly valves such as disclosed in U.S. Pat. No. 2,994,342 issued to Stillwagon, especially in pipe systems requiring a large internal diameter, are replacing the more conventional gate valve, primarily because they are more economical from a cost standpoint. Economies are realized both from a reduction of the material necessary to construct the butterfly valve and its lower shipping weight. Further, in some models, elastomer seats on opposite sides of the valve form an integral gasket enabling such a butterfly valve to be inserted between a pair of aligned pipe flanges which are thereafter pulled together with the flange bolts to form a leak-free joint between the valve and abutting ends of the pipe flanges.
Because the butterfly valve is essentially a cylindrical matching portion of the pipe system with a rotatable circular valve disc mounted therein so it can be oriented transverse to the flow axis to close off the flow through the valve and parallel therewith to allow full flow, it is highly compact, as part of the room for the disc swing is provided by the pipe line. In addition, only one quarter turn of the valve shaft supporting the valve disc is required to open or close the conventional butterfly valve, allowing rapid adjustment to different flow conditions.
With all of the indicated advantages, a properly constructed conventional butterfly valve, having a good elastomer seat for the disc and suitable seals for its valve shaft, does have some drawbacks. First, the torque necessary to rotate the valve shaft controlling the valve disc varies markedly with service conditions, and can, as an example, on a 12 inch butterfly valve, vary from a few foot pounds to over 310 foot pounds when operating a flow system containing it at 150 psi, as the disc is opened and/or closed. Further, the rate of change of torque is so rapid that an operator can easily lose control of the valve shaft, allowing the disc to "snap shut" or "open rapidly". In liquid systems having relatively high flow rates, loss of control of the valve disc can result in serious damage to the pipe system, including a rupture of the pipe system due to hydraulic surge following the rapid closing or opening of the butterfly valve. Therefore, the initial economy of the butterfly valve is often offset by the potential damage to the pipe system resulting from improper valve control. Thus, the safer gate valve is often used in its place to avoid such problems, since equipping the butterfly valve with a worm gear drive results in a cost approaching the cost of a gate valve.
Another disadvantage of the conventional butterfly valve occurs because of imprecise throttling control. Since the valve shaft turns only 90.degree. from full open to full close, precise control of the disc in all of its possible intermediate positions is required to match the control afforded by other types of valves, such as the gate valve. Conventional lever control of the valve shaft of a butterfly valve with limited "setting notches" therefore leaves a great deal to be desired in many fluid control systems.
Most of the advantages of conventional butterfly valves can be maintained if worm gear control of the valve shaft is employed. However, the prior art worm gear systems, with the drive shaft being oriented perpendicularly to the valve shaft, are too expensive and cumbersome, making the butterfly valve so equipped generally uncompetitive with the gate valve.
Thus the instant invention has as its object improved gear control of the valve shaft of a butterfly valve which is inexpensive and yet provides precise control of the valve disc without the appendant dangers of loss of control.